Changelog utils CompareCommits call only fetches 300 files

[jbohanon]

This can cause the changelog-bot to fail to find a valid changelog if there are >300 modifications that are alphanumerically prior to the location of the changelog file. This is in practice very unlikely to occur, but I have run into it while trying to restructure the changelog directories to put old versions' changelogs into minor-version subdirectories instead of patch-version subdirectories. This entailed moving >750 changelog files, all of which are alphanumerically prior to the changelog for the current PR.

I verified the 300 number by making the curl request against the API with

curl https://api.github.com/repos/solo-io/solo-projects/compare/v1.8.x...backports%2Fv1.8.x%2Fwaf-audit-logs-json -H "Accept: application/vnd.github+json" -H "Authorization: token <token with permission to view the repo>" -o ~/ghoutput